1. Field of the Invention
The present invention relates to a technology involving micromirror devices. More particularly, this invention relates to micromirror devices using mirror elements each controlled with a single address electrode.
2. Description of the Related Art
A micromirror is a microscopic mirror used in reflecting light. A digital micromirror device (DMD), that is, one composed of display elements implemented with a micro electromechanical system (MEMS) device configuration where an electric circuit is integrated on a silicon substrate and many micromirrors are arranged on the flat surface of the substrate, is generally known as a device using micromirrors. One can change the deflection angle of a micromirror surface on a conventional DMD by applying a voltage to two address electrodes positioned below each micromirror to generate a coulomb force F. Note that “deflection” referred to in this specification indicates the tilt of a micromirror surface. FIG. 1 is a circuit diagram that shows two driving circuits configured with memory cells with an SRAM configuration, which are connected to two address electrodes according to the conventional method.
A DMD is composed primarily of a substrate, a plurality of micromirrors, address electrodes used in deflecting the angle of the micromirrors, and elastic hinges for supporting each of the micromirrors. The elastic hinges are arranged on the substrate to support the micromirrors. In one mirror element, two address electrodes are situated immediately below each micromirror on the substrate. These address electrodes are connected to an external circuit via driving circuits on the substrate, whereby voltages are applied to the address electrodes.
U.S. Pat. No. 5,285,407 discloses the driving circuits configured for the two address electrodes in one mirror element of a DMD. To control the micromirror of the mirror element, the deflection of each micromirror is achieved by generating coulomb Forces F between the micromirror and the address electrodes, with voltages applied to the address electrodes via the driving circuits. Since the elastic hinges support the micromirrors, the flat surface of each micromirror is normally held in the position of the initial state by the restoring force of each elastic hinge when there is no control voltage applied to the electrodes.
U.S. Pat. No. 5,214,420 discloses the projection system using the above-described DMD. This projection system controls light by reflecting incident light on or away from a projection optical path by deflecting each micromirror as described above. In this patented disclosure, the light almost entirely reflected on a projection optical path, and light reflected away from the projection optical path are referred to as ON light and OFF light respectively. Additionally, light partially reflected on the projection optical path, which is light reflected on the projection optical path at a particular ratio of the ON light to the OFF light, that is, light the quantity of which is smaller than that of the ON light is referred to as intermediate light in this specification.
However, DMDs employed in the configurations disclosed by the above-described patents require two address electrodes for one mirror element. Therefore, it is necessary to connect two driving circuits to the two address electrodes. This leads to the technical problem of the relatively large surface area taken up by the driving circuits connected to the address electrodes on the substrate. Accordingly, when many mirror elements must be arranged on the substrate in order to obtain an image with a high-definition resolution, such as hi-vision, the area occupied by the driving circuits on the substrate expands with an increase in the number of mirror elements, leading to an increase in the size of the substrate itself. As a result, the projection apparatus becomes larger and more expensive.
U.S. Pat. No. 6,266,178 discloses the driving circuit 21 connected to an address electrode on a substrate by using a memory cell with a DRAM configuration as a charge storage cell. Namely, U.S. Pat. No. 6,266,178 discloses the invention for downsizing the driving circuit 21 by using the charge storage cell 20 shown in FIG. 2A.
FIG. 2B shows the configuration of the two driving circuits 21 that use the charge storage cell 20 and are connected to the two address electrodes in one mirror element. However, in this scenario, the technical problem described above, in that the size of the substrate itself must be increased due to the large area occupied by the driving circuits on the substrate when many mirror elements are arranged to obtain an image with a high-definition resolution, remains.
U.S. Pat. No. 6,975,444 discloses the configuration for controlling a mirror element with one address electrode with a DRAM or SRAM configuration. This document discloses the embodiment where a mirror connected to a cover glass is deflected in only one direction with respect to a substrate.
U.S. Pat. No. 6,885,494 discloses the technology related to the deflection angle of a mirror element. U.S. Pat. No. 4,229,732 discloses the configuration where a hinge is situated on a mirror surface. U.S. Pat. No. 5,061,049 discloses the landing electrode with the same potential as a mirror. U.S. Pat. No. 5,671,083 discloses the memory configuration using a capacitor. U.S. Pat. No. 6,657,759 discloses the technology for holding a mirror at a predetermined angle.
As described in these Patents above, in a DMD where two address electrodes are provided on a substrate in one mirror element, the two driving circuits required for each of the two address electrodes occupy a large area on the substrate because the configuration of the two driving circuits corresponding to the two address electrodes is required. This imposes a severe restriction on the arrangement of a larger number of mirror elements on the substrate. Additionally, previous to this invention, there have been no methods for controlling one mirror element with a single address electrode in a micromirror device. Furthermore, there have been no methods disclosed for controlling a mirror element to deflect in two directions with high precision.